1. Field of the Invention
The present invention is directed to a single mode optical waveguide fiber designed for long repeater spacing, high data rate telecommunication systems. In particular, the single mode waveguide combines excellent bend resistance, low attenuation, low dispersion and low dispersion slope, features that are desired for long distance transmission applications.
2. Technical Background
The requirement in the telecommunication industry for greater information capacity over long distances, without electronic signal regeneration, has led to a reevaluation of single mode fiber index profile design.
Recent developments in erbium-doped fiber amplifiers (EDFAs) and wavelength division multiplexing have enabled high-capacity lightwave systems. In order to achieve high capacity, channel bit rate and signal wavelength ranges can be increased. When bit rate is increased beyond 2.5 Gb/s, fiber dispersion has been a major degradation for long distance. On the other hand, if the dispersion is too low, multi-channel interactions can cause four-wave mixing and degrade system performance. In order to reduce both the dispersion and FWM degradations, dispersion management has been proposed and demonstrated. Dispersion management can be achieved by both cable management where +D and xe2x88x92D fibers are spliced alternatively and fiber management where core canes with +D and xe2x88x92D properties are combined to draw into one fiber.
Thus far, dispersion managed fibers using +D and xe2x88x92D fibers with positive dispersion slope have been proposed wherein the final fiber dispersion has a dispersion and slope similar to dispersion-shifted fiber, in other words, net zero dispersion is in the 1550 nm window and the total dispersion slope is positive. However, there is still a need for alternative designs of dispersion managed waveguides.
Definitions
The following definitions are in accord with common usage in the art. The index of refraction profile is defined in terms of the radii of segments of similar refractive indices. A particular segment has a first and a last refractive index point. The radius from the waveguide centerline to the location of this first refractive index point is the inner radius of the core region or segment. Likewise, the radius from the waveguide centerline to the location of the last refractive index point is the outer radius of the core segment.
The segment radius may be conveniently defined in a number of ways, as will be seen in the description of FIG. 1 below. In FIGS. 1-3, from which Tables 1 and 2 are derived, the radii of the index profile segments are defined as follows, where the reference is to a chart of xcex94 % vs. waveguide radius:
the outer radius of central major index profile, r1, is measured from the axial centerline of the waveguide to the intersection of the extrapolated central index profile with the x axis, i.e., the xcex94 %=0 point;
the outer radius, r2, of the first annular segment is measured from the axial centerline of the waveguide to the intersection of the extrapolated or actual central index profile with the x axis, i.e., the xcex94 %=0 point;
the outer radius, r3, of the second annular segment is measured from the axial centerline of the waveguide to the intersection of the extrapolated central index profile with the x axis, i.e., the xcex94 %=0 point;
the outer radius of any additional annular segments is measured analogously to the outer radii of the first and second annular segments; and
the radius of the final annular segment is measured from the waveguide centerline to the midpoint of the segment.
The width, w, of a segment is taken to be the distance between the inner and outer radius of the segment. It is understood that the outer radius of a segment corresponds to the inner radius of the next segment.
The relative index, xcex94, is defined by the equation,
xcex94 %=100xc3x97(n12xe2x88x92n22)/2n12,
where n1 is the maximum refractive index of the index profile segment 1, and n2 is a reference refractive index which is taken to be, in this application, the refractive index of the clad layer.
The term refractive index profile or simply index profile is the relation between xcex94 % or refractive index and radius over a selected portion of the core.
The term xcex1-profile refers to a refractive index profile expressed in terms of xcex94 (b) %, where b is radius, which follows the equation,
xcex94(b) %=xcex94(b0)(1xe2x88x92[|bxe2x88x92bo|/(b1xe2x88x92bo)]xcex1),
where bo is the radial point at which the index is a maximum and b1 is the point at which xcex94(b) % is zero and b is in the range bixe2x89xa6bxe2x89xa6bf, where delta is defined above, bi is the initial point of the xcex1-profile, bf is the final point of the xcex1-profile, and xcex1 is an exponent which is a real number.
Other index profiles include a step index, triangular, trapezoidal, and rounded step index, in which the rounding is typically due to dopant diffusion in regions of rapid refractive index change.
Total dispersion is defined as the algebraic sum of waveguide dispersion and material dispersion. Total dispersion is sometimes called chromatic dispersion in the art. The units of total dispersion are ps/nm-km.
The bend resistance of a waveguide fiber is expressed as induced attenuation under prescribed test conditions. Standard test conditions include 100 turns of waveguide fiber around a 75 mm diameter mandrel and 1 turn of waveguide fiber around a 32 mm diameter mandrel. In each test condition the bend induced attenuation, usually in units of dB/(unit length), is measured. In the present application, the bend test used is 5 turns of the waveguide fiber around a 20 mm diameter mandrel, a more demanding test which is required for the more severe operating environment of the present waveguide fiber.
One aspect of the present invention relates to a single mode optical waveguide comprising a first fiber component segment having a positive dispersion and a positive dispersion slope, and a second fiber component segment which has a negative dispersion and a negative dispersion slope, wherein the waveguide alternates along its length between segments of the first fiber component and the second fiber component, and wherein the first fiber component segment has a length which is at least two times the length of the second fiber component segment. The waveguide is optimized for the lower attenuation operating wavelength window around 1550 nm, i.e., in the window between about 1520 to 1625 nm.
The waveguide in accordance with the invention may be comprised of a unitary fiber having the various first and second segments therein, e.g., alternating sections of positive and negative dispersion and dispersion slope. Alternatively, the waveguide may be comprised of a cable in which the various fiber component sections are connected along the length of the cable.
Another aspect of the present invention relates to a single mode optical waveguide which manages fiber chromatic dispersion by providing a small total dispersion and a low dispersion slope. Preferred waveguides in accordance with the invention exhibit a dispersion over the range of 1520 to 1625 nm which at all times has a magnitude which is less than 2, and more preferably is less than 1 ps/nm-km. The total dispersion of the waveguide fiber is in the range of about xe2x88x922.0 to +2.0, more preferably about xe2x88x921.0 to +1.0, and most preferably about xe2x88x920.5 to +0.5 ps/nm-km at 1550 nm. The i1, xcex94i %, and the refractive index profiles of the various positive and negative dispersion segments are also selected to provide a total attenuation at 1550 nm no greater than 0.25 dB/km.
All of these properties are achieved while maintaining high strength, good fatigue resistance, and good bend resistance, i.e., an induced bend loss no greater than about 0.5 dB, for 1 turn about a 32 mm mandrel, and no greater than 0.05 dB for 100 turns around a 75 mm mandrel. The waveguides in accordance with the invention are also compatible with optical amplifiers. Also, cut off wavelength of fiber in cabled form is less than 1520 nm. An added benefit is a polarization mode dispersion less than about 0.5 ps/(km)1/2, more preferably less than 0.3 ps/(km)1/2 and typically about 0.1 ps/(km)1/2.
Additional features and advantages of the invention will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from that description or recognized by practicing the invention as described herein, including the detailed description which follows, the claims, as well as the appended drawings.
It is to be understood that both the foregoing general description and the following detailed description are merely exemplary of the invention, and are intended to provide an overview or framework for understanding the nature and character of the invention as it is claimed. The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate various embodiments of the invention, and together with the description serve to explain the principles and operation of the invention.